<!doctype html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>自适应的椭圆</title>
  <style>
    .round-box {
      width: 100px;
      height: 100px;
      background: #ffbb33;
      margin: 20px;
    }
    .box1 {
      border-radius: 50% / 100% 100% 0 0;
    }
    .box2 {
      border-radius: 100% 0 0 100% / 50%;
    }
    .box3 {
      border-radius: 100% 0 0 0;
    }
  </style>
</head>
<body>
  <div class="round-box box1"></div>
  <div class="round-box box2"></div>
  <div class="round-box box3"></div>
</body>
</html>
